1. Field of the Invention
The present invention relates to a method of operating an electrophotographic printing apparatus, and particularly to a method of operating an electrophotographic printing apparatus employing a reversal development system with a two-component developer which is generally used for a printer or a copying machine.
2. Description of the Prior Art
FIG. 1 shows the whole configuration of an electrophotographic printing apparatus employing a reversal development system with a two-component developer. In FIG. 1, the electrophotographic printing apparatus is constituted by a photosensitive body 1 and six units disposed around the photosensitive body 1, the six units being a charging unit 2, an exposing unit 3, a developing unit 4, a transferring unit 5, a charge eraser or an erasering unit 6, and a cleaning unit 7.
The photosensitive body 1 is a photoconductive body arranged to rotate at a constant speed. The charging unit 2 is constituted by a high voltage power supply 8 and a charger 9. The charger 9 is supplied with a high voltage from the high voltage power supply 8 so as to generate corona discharge to thereby make the photosensitive body 1 maintain its surface at a fixed potential, that is, a fixed surface potential V.sub.O. The exposing unit 3 radiates light onto the charged photosensitive body 1 so as to form an electrostatic latent image on the photosensitive body 1.
The developing unit 4 is constituted by a developer 12 which is a mixture of a toner 10 and a carrier 11, a developing roll 13, a bias voltage supply 14, and a bias voltage variable resistor 15. The toner 10, which is powder ink, is charged by friction with the carrier 11. The developing roll 13, which is a rotating roll, acts to convey the developer 12. The bias voltage supply 14 applies a bias voltage V.sub.B to the developing roll 13. The electrostatic latent image on the photosensitive body 1 is developed by toner 10 in accordance with the potential difference V.sub.O -V.sub.R between the surface potential V.sub.O on the photosensitive body 1 and the residual potential V.sub.R which is a potential on the photosensitive body 1 at the electrostatic latent image portion thereof, and the potential difference V.sub.B -V.sub.R between the bias voltage V.sub.B applied to the developing roll 13 and the residual potential V.sub.R. The bias voltage variable resistor 15 varies the bias voltage V.sub.B applied to the developing roll 13 so as to change the quantity of the toner 10 for developing the electrostatic latent image.
The transferring unit 5 acts to transfer the developing toner 10 from the surface of the photosensitive body 1 onto the printing paper 16. The charge eraser 6 lowers the potential of the photosensitive body 1 to the vicinity of zero volt so that the following electrophotographic process can be started. The cleaning unit 7 cleans out the toner 10 which has not been transferred onto the printing paper 16 so as to remain on the surface of the photosensitive body 1.
In a method of changing a printing density with the above conventional printing apparatus, the printing density is so changed by changing the position of the bias voltage variable resistor 15 as to change the potential difference V.sub.B -V.sub.R between the bias voltage V.sub.B of the developing roll 13 and the residual potential V.sub.R at the electrostatic latent image portion on the photosensitive body 1. In FIG. 2 is shown the relationship between the printing density and the potential difference V.sub.B -V.sub.R between the bias voltage V.sub.B and the residual potential V.sub.R.
However, problems in printing quality on the printing paper, for example, such as line width, field stains (so-called bronzing), and carrier stick (poor transfer due to sticking of the carrier 11 on the surface of the photosensitive body 1), are relative to the potential difference V.sub.O -V.sub.B between the surface potential V.sub.O of the photosensitive body 1 and the bias voltage V.sub.B of the developing roll 13. There is shown in FIG. 3 the relationship among the potential difference V.sub.B -V.sub.R between the bias voltage V.sub.B and the residual potential V.sub.R, the potential difference V.sub.O -V.sub.B between the surface potential V.sub.O and the bias voltage V.sub.B, the printing density, the line width, the field stains, the carrier stick. Since the line width and field stains are opposite in factor to the carrier stick, there has been a drawback that it is impossible to set the variable range of the printing density wide enough.